Plastics are composed of a variety of polymers such as acrylics, styrenes, phenolics, vinyls, esters, amides, ureas, carbonates, silicones, epoxies, ethylene, propylene, and cellulose derivatives. Each of these polymers has specific properties that are fairly unique for the material and its intended uses. Most plastic products or components are made from virgin plastic feedstocks, but in recent times, some products and components are being made from recycled plastics. The percentage of plastics in mixed wastes, such as MSW, has shown and is expected to continue to show a gradual increase with time. This increase in percentage is due in part to the reduced energy use, simplicity of manufacturing, and the lower costs of raw materials and labor when compared to similar items made of metals or glass. As a corollary, an increasing quantity of products and components are being made from mixtures of recycled plastics which yield a composite plastic material with its own unique properties and acceptance in a variety of applications.
Heretofore, methods of treatment of mixed wastes, such as MSW, have included disposal in sanitary landfills, mass incineration, preparation and combustion of refuse derived fuel (RDF), and size reduction for composting. Each of these methods results in the loss of the plastics by either burying, blending with soil, or combustion, and each has undesirable consequences. Plastics treated in landfills or composting are not really lost, but become nearly permanent components of the disposal site due to the chemical resistance and nonbiodegradable nature of these polymers. ln the combustion of plastics, hydrocarbons, dioxins, furans, acid gases, and NO.sub.x emissions in the stack gases of mass burn or RDF incineration facilities can be major sources of air pollution with rather severe health and environmental consequences unless stringent pollution control measures are observed.
Heretofore, the recycling of plastics has been primarily practiced by the plastic manufacturers themselves by the recycling of manufactured rejects into the virgin feedstocks. Source separation and recycling of certain mixed waste components, such as aluminum, glass, newspapers, and cardboard have been implemented to some extent by individuals, organizations, businesses, small communities, and in a few instances, even large cities. Plastics, however, have not been a major recycling component for source separation partly because of a lack of public awareness of the diversity of plastics and partly because of a lack of markets and economic incentives for recycled plastics. Even with the recent changes in the potential for source separation and recycling of plastics, education of the public and development of marketing networks for small scale recycling efforts may be long term. Source separation even in the best examples has resulted in only about a 30% reduction in the waste stream reaching disposal sites.
In my prior U.S. Pat. No. 4,342,830, dated Aug. 3,1982, steam treatment of MSW is disclosed wherein the sterilized and softened organics are forced through perforations upon sudden release of pressure whereby inorganics and certain synthetics, such as metal cans, glass, and the like are left behind. This method included the addition of significant quantities of water to the wastes and thus required substantial energy consumption in the form of steam to heat the water/waste slurry and to force the resultant sterile and softened organics through the perforations in a singular step. The internal steam pressure requirements of 20 to 100 psig and the resultant temperature range of approximately 228.degree. to 328.degree. F. for periods of 20 to 120 minutes also caused a melting of many plastics and significant heat distortion of most other plastics. Even at the minimum time of 20 minutes at the minimum steam pressure of 20 Psig, many of the low density plastic foams and films are melted and co-mingled with fabrics, paper, etc. making separation virtually impossible. The plastics thus recovered which were suitable for recycling were generally less than 2% of the waste by weight and volume and were generally severely contaminated with softened organics and dirt. The inorganic materials such as ferrous and non-ferrous metals were also similarly contaminated making these products less desirable for recycling. The glass components were both broken and contaminated.
In my prior U.S. Pat. No. 4,540,495 dated 10, 1985, steam treatment was again disclose wherein the amount of water added prior to steam treatment was significantly reduced to conserve the energy consumed and to reduce the moisture content of the softened organic fraction to only 60 to 70% by weight. The internal steam pressure requirements of 40 to 75 psig with the resultant temperature range of 270 to 320.degree. F. for periods of 30 to 90 minutes to cook and sterilize the waste material and soften the organic matter also caused a melting of many plastics and significant heat distortion of most other plastics as in the previous U.S. Pat. No. 4,342,830. However, in the case of U.S. Pat. No. 4,540,495 the contents of the process unit were separated externally into various components. Again, at the minimum time of 30 minutes at the minimum steam pressure of 40 psig, the low density plastics in particular are melted and co-mingled with other components. The severity of melting plastics is actually more extensive due to the higher pressure and longer time than in the earlier patent. The plastics recovered in this manner were more suitable for recycling, but generally only about 2% of these waste by weight and volume was recoverable as plastics in this manner. The inorganic materials such as ferrous and non-ferrous metals were also more suitable for recycling. The glass components were also cleaner, but were broken, making separation and recovery more difficult.
As much as 8% by weight of MSW today is plastic, and plastic may account for 10 to 15% of MSW by weight by the year 2000, making it the fastest growing component of MSW. Thus, it is both economically and environmentally sound that methods be devised to facilitate separation, recovery, and recycling of a greater quantity of plastic from mixed wastes, such as MSW, than has been possible with the methods disclosed in either U.S. Pat. No. 4,342,830 or U.S. Pat. No. 4,540,495.